Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP LTE systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system may support a time division duplex (TDD) and/or frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beam-forming gain on the forward link when multiple antennas are available at the access point.
In modern communication systems, timing is an important consideration and may be used to synchronize communications amongst a plurality of users, particularly in a synchronous OFDMA OR SC-FDM system. Base stations or Access Points (AP) may control the timing of mobile units or Access Terminals (AT) in order to mitigate possible interference caused to access terminals in the Return Link (RL) area/sector.
In TDD systems, each AT(s) may be positioned at different distances from the AP. Thus, an OFDM waveform from each client or AT may arrive at the AP at different instances in time. However, in TDD systems, each client or AT RL transmission may need to be aligned in time when received at the AP. Thus, if each RL transmission is not subsequently aligned at the AP (from a time perspective), then each client or AT may be creating interference with each other and the AP will not be able to decode any of the clients. Furthermore, since the Forward Link (FL) transmission may occur after the RL transmission in a TDD system, a delay in signals received at the AP may result in interference with the FL transmission.
In a TDD system, there are numerous silence intervals between subsequent AT transmissions where the AT is not transmitting data. Further, guard intervals are also utilized between the Forward Link (FL) and RL transmissions where no data transmission also occurs. Therefore, each AT may be able to advance/retard (or backoff) the RL or FL transmission such that it occurs earlier (or later) than expected (in time) in order to arrive synchronized at the AP. This concept is known as time retardation/advancement. Currently, the time retardation/advancement is only performed for TDD systems. However, in most FDD systems, the AT(s) transmit signals on the RL and FL continuously, resulting in little or no guard interval. Therefore, timing/synchronization is an unresolved issue in FDD systems. As a result, a need exists to account for timing adjustments in a FDD system. More specifically, a need exists for how the AT adjusts the RL timing in FDD systems.